Process for producing aerogels or xerogels

ABSTRACT

The present invention relates to a process for producing porous materials in the form of aerogels or xerogels, which comprises reaction of at least one polyfunctional isocyanate with an amine component comprising at least one polyfunctional substituted aromatic amine and also water in the presence of a solvent and a catalyst, with the reaction being carried out in the absence of water. 
     The invention further relates to the aerogels and xerogels which can be obtained in this way and the use of the aerogels and xerogels as insulation material and in vacuum insulation panels.

The present invention relates to a process for producing porous materials in the form of aerogels or xerogels, which comprises reaction of at least one polyfunctional isocyanate with an amine component comprising at least one polyfunctional substituted aromatic amine in the presence of a solvent and a catalyst, with the reaction being carried out in the absence of water.

The invention further relates to the aerogels and xerogels which can be obtained in this way and the use of the aerogels and xerogels as insulation material and in vacuum insulation panels.

Porous materials, for example polymer foams, having pores in the size range of a few microns or significantly below and a high porosity of at least 70% are particularly good thermal insulators on the basis of theoretical considerations.

Such porous materials having a small average pore diameter can be, for example, in the form of organic xerogels. In the literature, the term xerogel is not used entirely uniformly. In general, a xerogel is considered to be a porous material which has been produced by a sol-gel process, with the liquid phase having been removed from the gel by drying below the critical temperature and below the critical pressure of the liquid phase (“subcritical conditions”). In contrast, the term aerogels generally refers to gels obtained when the removal of the liquid phase from the gel has been carried out under supercritical conditions.

In the sol-gel process, a sol based on a reactive organic gel precursor is first produced and the sol is then gelled by means of a crosslinking reaction to form a gel. To obtain a porous material, for example a xerogel, from the gel, the liquid has to be removed. This step will hereinafter be referred to as drying in the interests of simplicity.

WO-95/02009 discloses isocyanate-based xerogels which are particularly suitable for applications in the field of vacuum insulation. The publication also discloses a sol-gel-based process for producing the xerogels, in which known, inter alia aromatic, polyisocyanates and an unreactive solvent are used. As further compounds having active hydrogen atoms, use is made of aliphatic or aromatic polyamines or polyols. The examples disclosed in the publication comprise ones in which a polyisocyanate is reacted with diaminodiethyltoluene. The xerogels disclosed generally have average pore sizes in the region of 50 μm. In one example, mention is made of an average pore diameter of 10 μm.

WO-2008/138978 discloses xerogels which comprise from 30 to 90% by weight of at least one polyfunctional isocyanate and from 10 to 70% by weight of at least one polyfunctional aromatic amine and have a volume average pore diameter of not more than 5 microns.

The unpublished EP-A 09178783.8 describes porous materials based on polyfunctional isocyanates and polyfunctional aromatic amines, where the amine component comprises polyfunctional substituted aromatic amines. The porous materials described are produced by reacting isocyanates with the desired amount of amine in a solvent which is inert toward the isocyanates. The formation of urea linkages occurs exclusively by reaction of the isocyanate groups with the amino groups used. The reaction proceeds without catalysis.

However, the materials properties, in particular the mechanical stability and/or the compressive strength and also the thermal conductivity, of the known porous materials based on polyurea are not satisfactory for all applications. In addition, the formulations on which the materials are based display shrinkage, with reduction of the porosity and an increase in the density, on drying. Furthermore, the gelling time, i.e. the time required for gelling of the starting compounds, is often too long. In addition, the average pore size of the known porous materials having a low density is often too high. Particularly at pressures above the vacuum range, e.g. in the range from 1 to 1000 mbar, a smaller average pore size is advantageous in terms of a favorable thermal conductivity.

A particular problem associated with the formulations based on isocyanates and amines which are known from the prior art are mixing defects. Mixing defects occur as a result of the high reaction rate between isocyanates and amino groups, since the gelling reaction has already proceeded a long way before complete mixing. Mixing defects lead to aerogels or xerogels having heterogeneous and unsatisfactory materials properties. A concept for reducing the phenomenon of mixing defects is thus generally desirable. However, an excessively long gelling time, i.e. an excessively long time until complete gelling of the gel precursor has occurred, should be avoided.

It was therefore an object of the invention to avoid the abovementioned disadvantages. In particular, an aerogel or xerogel which does not have the abovementioned disadvantages, or has them to a reduced extent, should be provided. The porous materials in the form of aerogels or xerogels should, compared to the prior art, have improved thermal-conductivity in vacuo. In addition, the aerogels and xerogels should have a low thermal conductivity even at pressures above the vacuum range, in particular in a pressure range from about 1 mbar to about 100 mbar. This is desirable since an increase in pressure occurs over time in vacuum panels. A further object of the present invention was to provide a porous material having a low thermal conductivity after admission of air, i.e. in the region of 1000 mbar. Furthermore, the aerogel or xerogel should at the same time have a high porosity, a low density and a sufficiently high mechanical stability.

Finally, mixing defects and thus the heterogeneities in the structure and the materials properties of the porous materials formed in the reaction of the isocyanates with the amines should be avoided.

We have accordingly found the process of the invention and the aerogels and xerogels which can be obtained in this way.

The process of the invention for producing an aerogel or xerogel comprises reacting the following components (a1) and (a2) in the absence of water and in the presence of a solvent and of at least one catalyst:

(a1) at least one polyfunctional isocyanate,

(a2) at least one polyfunctional substituted aromatic amine (a2-s) having the general formula I

where R¹ and R² can be identical or different and are each selected independently from among hydrogen and linear or branched alkyl groups having from 1 to 6 carbon atoms and all substituents Q¹ to Q⁵ and Q¹′ to Q⁵′ are identical or different and are each selected independently from among hydrogen, a primary amino group and a linear or branched alkyl group having from 1 to 12 carbon atoms, where the alkyl group can bear further functional groups, with the proviso that

-   -   the compound having the general formula I comprises at least two         primary amino groups, where at least one of Q¹, Q³ and Q⁵ is a         primary amino group and at least one of Q¹′, Q³′ and Q⁵′ is a         primary amino group, and     -   Q², Q⁴, Q²′ and Q⁴′ are selected so that the compound having the         general formula I has at least one linear or branched alkyl         group, which can bear further functional groups, having from 1         to 12 carbon atoms in the α position relative to at least one         primary amino group bound to the aromatic ring,         optionally at least one further polyfunctional aromatic amine         (a2-u) which differs from the amines (a2-s) having the general         formula I to form the aerogels or xerogels of the invention.

Preferred embodiments may be found in the claims and the description. Combinations of preferred embodiments do not go outside the scope of the present invention. Preferred embodiments of the components used are described below.

The polyfunctional isocyanates (a1) will hereinafter be referred to collectively as component (a1). Analogously, the polyfunctional amines (a2) will hereinafter be referred to collectively as component (a2). It will be obvious to a person skilled in the art that the monomer components mentioned are present in reacted form in the porous material.

For the purposes of the present invention, the functionality of a compound is the number of reactive groups per molecule. In the case of the monomer component (a1), the functionality is the number of isocyanate groups per molecule. In the case of the amino groups of the monomer component (a2), the functionality is the number of reactive amino groups per molecule. A polyfunctional compound has a functionality of at least 2.

If mixtures of compounds having different functionalities are used as component (a1) or (a2), the functionality of the components is in each case given by the number average of the functionality of the individual compounds. A polyfunctional compound comprises at least two of the abovementioned functional groups per molecule.

According to the invention, the reaction proceeds in the absence of water. A person skilled in the art will be aware that traces of water in the solvent cannot be completely ruled out. Small amounts of water do not influence the present invention. Accordingly, the expression “absence of water” means that the total amount of water based on the total weight of the components (a1) and (a2) and water is below 0.1% by weight, in particular from 0 to 0.09% by weight, particularly preferably from 0 to 0.08% by weight.

Component (a1)

In the process of the invention, at least one polyfunctional isocyanate is reacted as component (a1).

In the process of the invention, the amount of component (a1) used is preferably from 20 to 80% by weight, in particular from 25 to 75% by weight, particularly preferably from 35 to 68% by weight, in each case based on the total weight of the components (a1) and (a2), which is 100% by weight.

Possible polyfunctional isocyanates are aromatic, aliphatic, cycloaliphatic and/or araliphatic isocyanates. Such polyfunctional isocyanates are known per se or can be prepared by methods known per se. The polyfunctional isocyanates can also be used, in particular, as mixtures, so that the component (a1) in this case comprises various polyfunctional isocyanates. Polyfunctional isocyanates which are possible as monomer building blocks (a1) have two (hereinafter referred to as diisocyanates) or more than two isocyanate groups per molecule of the monomer component.

Particularly suitable polyfunctional isocyanates are diphenylmethane 2,2′-, 2,4′- and/or 4,4′-diisocyanate (MDI), naphthylene 1,5-diisocyanate (NDI), tolylene 2,4- and/or 2,6-diisocyanate (TDI), 3,3′-dimethylbiphenyl diisocyanate, 1,2-diphenylethane diisocyanate and/or p-phenylene diisocyanate (PPDI), trimethylene, tetramethylene, pentamethylene, hexamethylene, heptamethylene and/or octamethylene diisocyanate, 2-methylpentamethylene 1,5-diisocyanate, 2-ethylbutylene 1,4-diisocyanate, pentamethylene 1,5-diisocyanate, butylene 1,4-diisocyanate, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI), 1,4- and/or 1,3-bis(isocyanatomethyl)cyclohexane (HXDI), cyclohexane 1,4-diisocyanate, 1-methylcyclohexane 2,4- and/or 2,6-diisocyanate and dicyclohexylmethane 4,4′-, 2,4′- and/or 2,2′-diisocyanate.

As polyfunctional isocyanates (a1), preference is given to aromatic isocyanates. Particularly preferred polyfunctional isocyanates of the component (a1) are the following embodiments:

-   -   i) polyfunctional isocyanates based on tolylene diisocyanate         (TDI), in particular 2,4-TDI or 2,6-TDI or mixtures of 2,4- and         2,6-TDI;     -   ii) polyfunctional isocyanates based on diphenylmethane         diisocyanate (MDI), in particular 2,2′-MDI or 2,4′-MDI or         4,4′-MDI or oligomeric MDI, also referred to as         polyphenylpolymethylene isocyanate, or mixtures of two or three         of the abovementioned diphenylmethane diisocyanates or crude MDI         which is obtained in the production of MDI or mixtures of at         least one oligomer of MDI and at least one of the abovementioned         low molecular weight MDI derivatives;     -   iii) mixtures of at least one aromatic isocyanate according to         embodiment i) and at least one aromatic isocyanate according to         embodiment ii).

Oligomeric diphenylmethane diisocyanate is particularly preferred as polyfunctional isocyanate. Oligomeric diphenylmethane diisocyanate (hereinafter referred to as oligomeric MDI) is an oligomeric condensation product or a mixture of a plurality of oligomeric condensation products and thus a derivative/derivatives of diphenylmethane diisocyanate (MDI). The polyfunctional isocyanates can preferably also be made up of mixtures of monomeric aromatic diisocyanates and oligomeric MDI.

Oligomeric MDI comprises one or more condensation products of MDI which have a plurality of rings and a functionality of more than 2, in particular 3 or 4 or 5. Oligomeric

MDI is known and is frequently referred to as polyphenylpolymethylene isocyanate or as polymeric MDI. Oligomeric MDI is usually made up of a mixture of MDI-based isocyanates having various functionalities. Oligomeric MDI is usually used in admixture with monomeric MDI.

The (average) functionality of an isocyanate comprising oligomeric MDI can vary in the range from about 2.2 to about 5, in particular from 2.4 to 3.5, in particular from 2.5 to 3. Such a mixture of MDI-based polyfunctional isocyanates having various functionalities is, in particular, crude MDI which is obtained in the production of MDI.

Polyfunctional isocyanates or mixtures of a plurality of polyfunctional isocyanates based on MDI are known and are marketed, for example, by BASF Polyurethanes GmbH under the name Lupranat®.

The functionality of the component (a1) is preferably at least two, in particular at least 2.2 and particularly preferably at least 2.5. The functionality of the component (a1) is preferably from 2.2 to 4 and particularly preferably from 2.5 to 3.

The content of isocyanate groups in the component (a1) is preferably from 5 to 10 mmol/g, in particular from 6 to 9 mmol/g, particularly preferably from 7 to 8.5 mmol/g. A person skilled in the art will know that the content of isocyanate groups in mmol/g and the equivalent weight in g/equivalent have a reciprocal relationship. The content of isocyanate groups in mmol/g can be derived from the content in % by weight in accordance with ASTM D-5155-96 A.

In a preferred embodiment, the component (a1) comprises at least one polyfunctional isocyanate selected from among diphenylmethane 4,4′-diisocyanate, diphenylmethane 2,4′-diisocyanate, diphenylmethane 2,2′-diisocyanate and oligomeric diphenylmethane diisocyanate. In this preferred embodiment, the component (a1) particularly preferably comprises oligomeric diphenylmethane diisocyanate and has a functionality of at least 2.5.

The viscosity of the component (a1) used can vary within a wide range. The component (a1) preferably has a viscosity of from 100 to 3000 mPa·s, particularly preferably from 200 to 2500 mPa·s.

Component (a2)

According to the invention, at least one polyfunctional substituted aromatic amine (a2-s) having the general formula I

where R¹ and R² can be identical or different and are each selected independently from among hydrogen and linear or branched alkyl groups having from 1 to 6 carbon atoms and all substituents Q¹ to Q⁵ and Q¹′ to Q⁵′ are identical or different and are each selected independently from among hydrogen, a primary amino group and a linear or branched alkyl group having from 1 to 12 carbon atoms, where the alkyl group can bear further functional groups, with the proviso that

-   -   the compound having the general formula I comprises at least two         primary amino groups, where at least one of Q¹, Q³ and Q⁵ is a         primary amino group and at least one of Q¹′, Q³′ and Q⁵′ is a         primary amino group, and     -   Q², Q⁴, Q²′ and Q⁴′ are selected so that the compound having the         general formula I has at least one linear or branched alkyl         group, which can optionally bear further functional groups,         having from 1 to 12 carbon atoms in the α position relative to         at least one primary amino group bound to the aromatic ring, and         optionally at least one further polyfunctional aromatic amine         (a2-u) which differs from the amines (a2-s) having the general         formula I is/are reacted as component (a2).

Component (a2) thus comprises polyfunctional aromatic amines, with the polyfunctional aromatic amines (a2-s) having the general formula I being a constituent.

For the purposes of the present invention, polyfunctional amines are amines which have at least two amino groups which are reactive toward isocyanates per molecule. Here, primary and secondary amino groups are reactive toward isocyanates, with the reactivity of primary amino groups generally being significantly higher than that of secondary amino groups.

The amount of component (a2) used is preferably from 20 to 80% by weight, in particular from 25 to 75% by weight, particularly preferably from 32 to 65% by weight, in each case based on the total weight of the components (a1) and (a2), which is 100% by weight.

According to the invention, R¹ and R² in the general formula I are identical or different and are each selected independently from among hydrogen, a primary amino group and a linear or branched alkyl group having from 1 to 6 carbon atoms. R¹ and R² are preferably selected from among hydrogen and methyl. Particular preference is given to R¹═R²═H.

Q², Q⁴, Q²′ and Q⁴′ are preferably selected so that the substituted aromatic amine (a2-s) comprises at least two primary amino groups which each have one or two linear or branched alkyl groups having from 1 to 12 carbon atoms, which may bear further functional groups, in the α position. If one or more of Q², Q⁴, Q²′ and Q⁴′ are selected so that they correspond to linear or branched alkyl groups which have from 1 to 12 carbon atoms and bear further functional groups, preference is given to amino groups and/or hydroxy groups and/or halogen atoms as such functional groups.

The alkyl groups as substituents Q in the general formula I are preferably selected from among methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl and tert-butyl.

The amines (a2-s) are preferably selected from the group consisting of 3,3′,5,5′-tetraalkyl-4,4′-diaminodiphenylmethane, 3,3′,5,5′-tetraalkyl-2,2′-diaminodiphenyl-methane and 3,3′,5,5′-tetraalkyl-2,4′-diaminodiphenylmethane, where the alkyl groups in the 3,3′,5 and 5′ positions can be identical or different and are each selected independently from among linear or branched alkyl groups which have from 1 to 12 carbon atoms and can bear further functional groups. The abovementioned alkyl groups are preferably methyl, ethyl, n-propyl, i-propyl, n-butyl, sec-butyl or t-butyl (in each case unsubstituted).

In one embodiment, one, more than one or all hydrogen atoms of one or more alkyl groups of the substituents Q can have been replaced by halogen atoms, in particular chlorine. As an alternative, one, more than one or all hydrogen atoms of one or more alkyl groups of the substituents Q can have been replaced by NH₂ or OH. However, the alkyl groups in the general formula I are preferably made up of carbon and hydrogen.

In a particularly preferred embodiment, component (a2) comprises 3,3′,5,5′-tetraalkyl-4,4′-diaminodiphenylmethane, where the alkyl groups can be identical or different and are each selected independently from among linear or branched alkyl groups which have from 1 to 12 carbon atoms and can optionally bear functional groups. The abovementioned alkyl groups are preferably selected from among unsubstituted alkyl groups, in particular methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl and tert-butyl, particularly preferably methyl and ethyl. Very particular preference is given to 3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane and/or 3,3′,5,5′-tetramethyl-4,4′-diaminodiphenylmethane.

The abovementioned polyfunctional amines of the type (a2-s) are known per se to those skilled in the art or can be prepared by known methods. One of the known methods is the reaction of aniline or derivatives of aniline with formaldehyde in the presence of an acid catalyst, in particular the reaction of 2,4- or 2,6-dialkylaniline.

The component (a2) can optionally also comprise further polyfunctional aromatic amines (a2-u) which differ from the amines of the structure (a2-s). The aromatic amines (a2-u) preferably have exclusively aromatically bound amino groups, but can also have both (cyclo)aliphatically and aromatically bound reactive amino groups.

Suitable polyfunctional aromatic amines (a2-u) are, in particular, isomers and derivatives of diaminodiphenylmethane. Isomers and derivatives of diaminodiphenylmethane which are preferred as constituents of component (a2) are, in particular, 4,4′-diaminodiphenylmethane, 2,4′-diaminodiphenylmethane, 2,2′-diaminodiphenylmethane and oligomeric diaminodiphenylmethane.

Further suitable polyfunctional aromatic amines (a2-u) are, in particular, isomers and derivatives of toluenediamine. Isomers and derivatives of toluenediamine which are preferred as constituents of component (a2) are, in particular, toluene-2,4-diamine and/or toluene-2,6-diamine and diethyltoluenediamines, in particular 3,5-diethyltoluene-2,4-diamine and/or 3,5-diethyltoluene-2,6-diamine.

In a first, particularly preferred embodiment, component (a2) consists exclusively of polyfunctional aromatic amines of the type (a2-s). In a second preferred embodiment, component (a2) comprises polyfunctional aromatic amines of the types (a2-s) and (a2-u). In the latter, second preferred embodiment, the component (a2) preferably comprises at least one polyfunctional aromatic amine (a2-u), of which at least one is selected from among isomers and derivatives of diaminodiphenylmethane (MDA).

In the second preferred embodiment, component (a2) correspondingly particularly preferably comprises at least one polyfunctional aromatic amine (a2-u) selected from among 4,4′-diaminodiphenylmethane, 2,4′-diaminodiphenylmethane, 2,2′-diaminodiphenylmethane and oligomeric diaminodiphenylmethane.

Oligomeric diaminodiphenylmethane comprises one or more methylene-bridged condensation products of aniline and formaldehyde having a plurality of rings. Oligomeric MDA comprises at least one oligomer, but in general a plurality of oligomers, of MDA having a functionality of more than 2, in particular 3 or 4 or 5. Oligomeric MDA is known or can be prepared by methods known per se. Oligomeric MDA is usually used in the form of mixtures with monomeric MDA.

The (average) functionality of a polyfunctional amine (a2-u) comprising oligomeric MDA can vary in the range from about 2.3 to about 5, in particular from 2.3 to 3.5 and in particular from 2.3 to 3. One such mixture of MDA-based polyfunctional amines having differing functionalities is, in particular, crude MDA which is formed, in particular, as intermediate in the condensation of aniline with formaldehyde, usually catalyzed by hydrochloric acid, in the production of crude MDI.

In the abovementioned preferred second embodiment, particular preference is given to the component (a2) comprising oligomeric diaminodiphenylmethane as compound (a2-u) and having an overall functionality of at least 2.1.

The proportion of amines of type (a2-s) having the general formula I based on the total weight of all polyfunctional amines of the component (a2), which thus add up to a total of 100% by weight, is preferably from 10 to 100% by weight, in particular from 30 to 100% by weight, very particularly preferably from 50 to 100% by weight, in particular from 80 to 100% by weight.

The proportion of polyfunctional aromatic amines (a2-u) which differ from the amines of type (a2-s) based on the total weight of all polyfunctional amines of the component (a2) is preferably from 0 to 90% by weight, in particular from 0 to 70% by weight, particularly preferably from 0 to 50% by weight, in particular from 0 to 20% by weight.

In the present invention, the calculated use ratio (equivalence ratio) n^(NCO)/n^(amine) can vary over a wide range and in particular be from 1.01 to 5. The equivalence ratio mentioned is particularly preferably from 1.1 to 3, in particular from 1.1 to 2. An excess of n^(NCO) over n^(amine) leads, in this embodiment, to lower shrinkage of the porous material, in particular xerogel, in the removal of the solvent and as a result of synergistic interaction with the catalyst to an improved network structure and to improved final properties of the resulting porous material.

In the present invention, the abovementioned components (a1) and (a2) are preferably used in the following ratio, in each case based on the total weight of the components (a1) and (a2), which is 100% by weight: from 20 to 80% by weight, in particular from 25 to 75% by weight, particularly preferably from 35 to 68% by weight, of the component (a1), from 20 to 80% by weight, in particular from 25 to 75% by weight, particularly preferably from 32 to 65% by weight, of the component (a2).

The components (a1) and (a2) will hereinafter be referred to collectively as organic gel precursor (A).

Catalyst

The process of the invention is preferably carried out in the presence of at least one catalyst as component.

Possible catalysts are in principle all catalysts known to those skilled in the art which accelerate the trimerization of isocyanates (known as trimerization catalysts) and/or the reaction of isocyanates with aromatically bound amino groups (known as gelling catalysts).

The corresponding catalysts are known per se and have different relative activities in respect of the abovementioned reactions. Depending on the relative activity, they can thus be assigned to one or more of the abovementioned types. Furthermore, it will be known to a person skilled in the art that reactions other than those mentioned above can also occur.

Corresponding catalysts can be characterized, inter alia, according to their catalytic activity, as is known, for example, from Polyurethane, 3^(rd) edition, G. Oertel, Hanser Verlag, Munich, 1993, page 104-110.

Preferred catalysts have a significant activity in respect of trimerization. This favorably influences the homogeneity of the network structure, resulting in particularly advantageous mechanical properties.

The catalysts can be able to be incorporated as a monomer building block (incorporatable catalyst) or not be able to be incorporated, with preference being given to catalysts which cannot be incorporated.

The component is advantageously used in the smallest effective amount. Preference is given to employing amounts of from 0.01 to 5 parts by weight, in particular from 0.1 to 3 parts by weight, particularly preferably from 0.2 to 2.5 parts by weight, of the catalyst or catalysts (referred to as component (a4) below) based on a total of 100 parts by weight of the components (a1) and (a2).

Catalysts preferred as component (a4) are selected from the group consisting of primary, secondary and tertiary amines, triazine derivatives, organic metal compounds, metal chelates, quaternary ammonium salts, ammonium hydroxides and also alkali metal and alkaline earth metal hydroxides, alkoxides and carboxylates.

Suitable catalysts are in particular strong bases, for example quaternary ammonium hydroxides such as tetraalkylammonium hydroxides having from 1 to 4 carbon atoms in the alkyl radical and benzyltrimethylammonium hydroxide, alkali metal hydroxides such as potassium or sodium hydroxide and alkali metal alkoxides such as sodium methoxide, potassium and sodium ethoxide and potassium isopropoxide.

Further suitable catalysts are, in particular, alkali metal salts of carboxylic acids, e.g. potassium formate, sodium acetate, potassium acetate, potassium 2-ethylhexanoate, potassium adipate and sodium benzoate, alkali metal salts of long-chain fatty acids having from 10 to 20 carbon atoms and optionally lateral OH groups.

Further suitable catalysts are, in particular, N-hydroxyalkyl quaternary ammonium carboxylates, e.g. trimethylhydroxypropylammonium formate.

Organic metal compounds as, in particular, gelling catalysts are known per se to those skilled in the art and are likewise suitable as catalysts (a4). Organic tin compounds such as tin 2-ethylhexanoate and dibutyltin dilaurate are preferred as constituents of component (a4).

Tertiary amines are known per se to those skilled in the art as gelling catalysts and as trimerization catalysts. Tertiary amines are particularly preferred as catalysts (a4). Preferred tertiary amines are, in particular, N,N-dimethylbenzylamine, N,N′-dimethylpiperazine, N,N-dimethylcyclohexylamine, N,N′,N″-tris(dialkylaminoalkyl)-s-hexahydrotriazines, such as N,N′,N″-tris(dimethylaminopropyl)-s-hexahydrotriazine, tris(dimethylaminomethyl)phenol, bis(2-dimethylaminoethyl)ether, N,N,N,N,N-pentamethyldiethylenetriamine, methylimidazole, dimethylbenzylamine, 1,6-diazabicyclo[5.4.0]undec-7-ene, triethylamine, triethylenediamine (IUPAC: 1,4-diazabicyclo[2.2.2]octane), dimethylaminoethanolamine, dimethylaminopropylamine, N,N-dimethylaminoethoxyethanol, N,N,N-trimethylaminoethylethanolamine, triethanolamine, diethanolamine, triisopropanolamine and diisopropanolamine.

Catalysts which are particularly preferred as component (a4) are selected from the group consisting of N,N-dimethylcyclohexylamine, bis(2-dimethylaminoethyl)ether, N,N,N,N,N-pentamethyldiethylenetriamine, methylimidazole, dimethylbenzylamine, 1,6-diazabicyclo[5.4.0]undec-7-ene, tris(dimethylaminopropyl)-s-hexahydrotriazine, triethylamine, tris(dimethylaminomethyl)phenol, triethylenediamine (diazabicyclo[2.2.2]octane), dimethylaminoethanolamine, dimethylaminopropylamine,

N,N-dimethylaminoethoxyethanol, N,N,N-trimethylaminoethylethanolamine, triethanolamine, diethanolamine, triisopropanolamine, diisopropanolamine, metal acetylacetonates, ammonium ethylhexanoates and metal ethylhexanoates.

The use of the catalysts which are preferred for the purposes of the present invention leads to aerogels and xerogels having smaller average pore diameters and improved mechanical properties, in particular improved compressive strength. In addition, the gelling time is reduced by use of the catalysts (a4), i.e. the gelling reaction is accelerated, without other properties being adversely affected.

Solvent

According to the present invention, the reaction takes place in the presence of a solvent.

For the purposes of the present invention, the term solvent comprises liquid diluents, i.e. both solvents in the narrower sense and also dispersion media. The mixture can, in particular, be a true solution, a colloidal solution or a dispersion, e.g. an emulsion or suspension. The mixture is preferably a true solution. The solvent is a compound which is liquid under the conditions of step (a), preferably an organic solvent.

The solvent can in principle be an organic compound or a mixture of a plurality of compounds, with the solvent being liquid under the temperature and pressure conditions under which the mixture is provided in step (a) (dissolution conditions for short). The composition of the solvent is selected so that it is able to dissolve or disperse, preferably dissolve, the organic gel precursor. Preferred solvents are those which are a solvent for the organic gel precursor (A), i.e. ones which dissolve the organic gel precursor (A) completely under the reaction conditions.

The reaction product of the reaction in the presence of the solvent is initially a gel, i.e. a viscoelastic chemical network which is swollen by the solvent. A solvent which is a good swelling agent for the network formed in step (b) generally leads to a network having fine pores and a small average pore diameter, while a solvent which is a poor swelling agent for the gel resulting from step (b) generally leads to a coarse-pored network having a large average pore diameter.

The choice of the solvent thus influences the desired pore size distribution and the desired porosity. The choice of the solvent is also generally made in such a way that precipitation or flocculation due to formation of a precipitated reaction product does not occur to a significant extent during or after step (b) of the process of the invention.

When a suitable solvent is chosen, the proportion of precipitated reaction product is usually less than 1% by weight, based on the total weight of the mixture. The amount of precipitated product formed in a particular solvent can be determined gravimetrically by filtering the reaction mixture through a suitable filter before the gelling point.

Possible solvents are the solvents known from the prior art for isocyanate-based polymers. Preferred solvents are those which are a solvent for the components (a1) and (a2), i.e. solvents which dissolve the constituents of the components (a1) and (a2) virtually completely under the reaction conditions. The solvent is preferably inert, i.e. unreactive, toward component (a1).

Possible solvents are, for example, ketones, aldehydes, alkyl alkanoates, amides such as formamide and N-methylpyrollidone, sulfoxides such as dimethyl sulfoxide, aliphatic and cycloaliphatic halogenated hydrocarbons, halogenated aromatic compounds and fluorine-containing ethers. Mixtures of two or more of the abovementioned compounds are likewise possible.

Further possibilities as solvent are acetals, in particular diethoxymethane, dimethoxymethane and 1,3-dioxolane.

Dialkyl ethers and cyclic ethers are likewise suitable as solvents. Preferred dialkyl ethers are, in particular, those having from 2 to 6 carbon atoms, in particular methyl ethyl ether, diethyl ether, methyl propyl ether, methyl isopropyl ether, propyl ethyl ether, ethyl isopropyl ether, dipropyl ether, propyl isopropyl ether, diisopropyl ether, methyl butyl ether, methyl isobutyl ether, methyl t-butyl ether, ethyl n-butyl ether, ethyl isobutyl ether and ethyl t-butyl ether. Preferred cyclic ethers are, in particular, tetrahydrofuran, dioxane and tetrahydropyran.

Aldehydes and/or ketones are preferred as solvents. Aldehydes or ketones suitable as solvents are, in particular, those corresponding to the general formula R²—(CO)—R¹, where R¹ and R² are each hydrogen or an alkyl group having 1, 2, 3 or 4 carbon atoms. Suitable aldehydes or ketones are, in particular, acetaldehyde, propionaldehyde, n-butyraldehyde, isobutyraldehyde, 2-ethylbutyraldehyde, valeraldehyde, isopentaldehyde, 2-methylpentaldehyde, 2-ethylhexaldehyde, acrolein, methacrolein, crotonaldehyde, furfural, acrolein dimer, methacrolein dimer, 1,2,3,6-tetrahydrobenzaldehyde, 6-methyl-3-cyclohexenaldehyde, cyanoacetaldehyde, ethyl glyoxylate, benzaldehyde, acetone, methyl isobutyl ketone, diethyl ketone, methyl ethyl ketone, methyl isobutyl ketone, methyl n-butyl ketone, ethyl isopropyl ketone, 2-acetylfuran, 2-methoxy-4-methylpentan-2-one, cyclohexanone and acetophenone. The abovementioned aldehydes and ketones can also be used in the form of mixtures. Ketones and aldehydes having alkyl groups having up to 3 carbon atoms per substituent are particularly preferred as solvents. Very particular preference is given to ketones of the general formula R¹(CO)R², where R¹ and R² are selected independently from among alkyl groups having from 1 to 3 carbon atoms. In a first preferred embodiment, the ketone is acetone. In a further preferred embodiment, at least one of the two substituents R¹ and/or R² comprises an alkyl group having at least 2 carbon atoms, in particular methyl ethyl ketones. Use of the abovementioned particularly preferred ketones in combination with the process of the invention gives aerogels and xerogels having a particularly small average pore diameter. Without applying restriction, it is presumed that the pore structure of the gel being formed is particularly fine-pored because of the greater affinity of the abovementioned particularly preferred ketones.

In many cases, particularly suitable solvents are obtained by using two or more completely miscible compounds selected from the abovementioned solvents in the form of a mixture.

To obtain a sufficiently stable gel which does not shrink too much during drying in step (c) in step (b), the proportion of the components (a1) and (a2) based on the total weight of the components (a1) and (a2) and the solvent, which is 100% by weight, must generally be not less than 5% by weight. The proportion of the components (a1) and (a2) based on the total weight of the components (a1) and (a2) and the solvent, which is 100% by weight, is preferably at least 6% by weight, particularly preferably at least 8% by weight, in particular at least 10% by weight.

On the other hand, the concentration of the components (a1) and (a2) in the mixture provided must not be too high since otherwise no porous material having favorable properties is obtained. In general, the proportion of the components (a1) and (a2) based on the total weight of the components (a1) and (a2) and the solvent, which is 100% by weight, is not more than 40% by weight. The proportion of the components (a1) and (a2) based on the total weight of the components (a1) and (a2) and the solvent, which is 100% by weight, is preferably not more than 35% by weight, particularly preferably not more than 25% by weight, in particular not more than 20% by weight.

The total proportion by weight of the components (a1) and (a2) based on the total weight of the components (a1) and (a2) and the solvent, which is 100% by weight, is preferably from 8 to 25% by weight, in particular from 10 to 20% by weight, particularly preferably from 12 to 18% by weight. Adherence to the amount of the starting materials in the range mentioned leads to aerogels and xerogels having a particularly advantageous pore structure, low thermal conductivity and low shrinking during drying.

Before the reaction, it is necessary to mix the components used, in particular to mix them homogeneously. The rate of mixing should be high relative to the rate of the reaction in order to avoid mixing defects. Appropriate mixing methods are known per se to those skilled in the art.

Preferred process for producing the aerogels and xerogels

In a preferred embodiment, the process of the invention comprises at least the following steps:

-   -   (a) provision of the components (a1) and (a2) the catalyst, and         the solvent as described above,     -   (b) reaction of the components (a1) and (a2) in the presence of         the solvent and the catalyst to form a gel and     -   (c) drying of the gel obtained in the preceding step.

Preferred embodiments of steps (a) to (c) will be described in detail below. Step (a)

According to the invention, the components (a1) and (a2) and the solvent are provided in step (a).

The components (a1) and (a2) are preferably provided separately from one another, each in a suitable partial amount of the solvent. The separate provision makes it possible for the gelling reaction to be optimally monitored or controlled before and during mixing.

The mixture or mixtures provided in step (a) can also comprise customary auxiliaries known to those skilled in the art as further constituents. Mention may be made by way of example of surface-active substances, flame retardants, nucleating agents, oxidation stabilizers, lubricants and mold release agents, dyes and pigments, stabilizers, e.g. against hydrolysis, light, heat or discoloration, inorganic and/or organic fillers, reinforcing materials and biocides.

Further information regarding the abovementioned auxiliaries and additives may be found in the specialist literature, e.g. in Plastics Additive Handbook, 5th edition, H. Zweifel, ed. Hanser Publishers, Munich, 2001.

The catalyst can in principle be added to one of the two components before mixing or to the resulting mixture. The catalyst is preferably added to the solvent in the presence of the component (a2).

Step (b)

According to the invention, the reaction of the components (a1) and (a2) takes place in the presence of the solvent to form a gel in step (b). To carry out the reaction, a homogeneous mixture of the components provided in step (a) firstly has to be produced.

The mixing of the components provided in step (a) can be carried out in a conventional way. A stirrer or another mixing device is preferably used here in order to achieve good and rapid mixing. The time required for producing the homogeneous mixture should be short in relation to the time during which the gelling reaction leads to at least partial formation of a gel, in order to avoid mixing defects. The other mixing conditions are generally not critical; for example, mixing can be carried out at from 0 to 100° C. and from 0.1 to 10 bar (absolute), in particular at, for example, room temperature and atmospheric pressure. After a homogeneous mixture has been produced, the mixing apparatus is preferably switched off.

The gelling reaction is a polyaddition reaction, in particular a polyaddition of isocyanate groups and amino groups.

For the purposes of the present invention, a gel is a crosslinked system based on a polymer which is present in contact with a liquid (known as Solvogel or Lyogel, or with water as liquid: aquagel or hydrogel). Here, the polymer phase forms a continuous three-dimensional network.

In step (b) of the process of the invention, the gel is usually formed by allowing to rest, e.g. by simply allowing the container, reaction vessel or reactor in which the mixture is present (hereinafter referred to as gelling apparatus) to stand. The mixture is preferably no longer stirred or mixed during gelling (gel formation) because this could hinder formation of the gel. It has been found to be advantageous to cover the mixture during gelling or to close the gelling apparatus.

Gelling is known per se to a person skilled in the art and is described, for example, in WO-2009/027310 on page 21, line 19 to page 23, line 13, the contents of which are hereby fully incorporated by reference.

Step (c)

According to the invention, the gel obtained in the previous step is dried in step (c).

Drying under supercritical conditions is in principle possible, preferably after replacement of the solvent by CO₂ or other solvents suitable for the purposes of supercritical drying. Such drying is known per se to a person skilled in the art. Supercritical conditions characterize a temperature and a pressure at which the fluid phase to be removed is present in the supercritical state. In this way, shrinkage of the gel body on removal of the solvent can be reduced.

However, in view of the simple process conditions, preference is given to drying the gels obtained by conversion of the liquid comprised in the gel into the gaseous state at a temperature and a pressure below the critical temperature and the critical pressure of the liquid comprised in the gel.

The drying of the gel obtained is preferably carried out by converting the solvent into the gaseous state at a temperature and a pressure below the critical temperature and the critical pressure of the solvent. Accordingly, drying is preferably carried out by removing the solvent which was present in the reaction without prior replacement by a further solvent.

Such methods are likewise known to those skilled in the art and are described in WO-2009/027310 on page 26, line 22 to page 28, line 36, the contents of which are hereby fully incorporated by reference.

Properties of the Aerogels and Xerogels and Use

The present invention further provides the aerogels and xerogels which can be obtained by the process of the invention.

Xerogels are preferred as porous materials for the purposes of the present invention, i.e. the porous material which can be obtained according to the invention is preferably a xerogel.

For the purposes of the present invention, a xerogel is a porous material which has a porosity of at least 70% by volume and a volume average pore diameter of not more than 50 microns and has been produced by a sol-gel process, with the liquid phase having been removed from the gel by drying below the critical temperature and below the critical pressure of the liquid phase (“subcritical conditions”).

The average pore diameter is determined by means of mercury intrusion in accordance with DIN 66133 and for the purposes of the present invention is essentially a volume average. The mercury intrusion method in accordance with DIN 66133 is a porosimetric method and is carried out in a porosimeter. Here, mercury is pressed into a sample of the porous material. Small pores require a higher pressure in order to be filled with the mercury than do large pores, and a pore size distribution and the volume average pore diameter can be determined from the corresponding pressure volume graph.

The volume average pore diameter of the porous material is preferably not more than 4 microns. The volume average pore diameter of the porous material is particularly preferably not more than 3 microns, very particularly preferably not more than 2 microns and in particular not more than 1 microns.

Although a very small pore size combined with a high porosity is desirable from the point of view of a low thermal conductivity, from the point of view of production and to obtain a sufficiently mechanically stable aerogel or xerogel, there is a practical lower limit to the volume average pore diameter. In general, the volume average pore diameter is at least 100 nm, preferably at least 150 nm. In many cases, the volume average pore diameter is at least 200 nm, in particular at least 300 nm.

The aerogel or xerogel which can be obtained according to the invention preferably has a porosity of at least 70% by volume, in particular from 70 to 99% by volume, particularly preferably at least 80% by volume, very particularly preferably at least 85% by volume, in particular from 85 to 95% by volume. The porosity in % by volume means that the specified proportion of the total volume of the aerogel or xerogel comprises pores. Although a very high porosity is usually desirable from the point of view of a minimal thermal conductivity, an upper limit is imposed on the porosity by the mechanical properties and the proccessability of the aerogel or xerogel.

The components (a1) and (a2) are present in reactive (polymer) form in the aerogel or xerogel which can be obtained according to the invention. Owing to the composition according to the invention, the monomer building blocks (a1) and (a2) are predominantly bound via urea linkages and/or via isocyanurate linkages in the porous material, with the isocyanurate groups being formed by trimerization of isocyanate groups of the monomer building blocks (a1). If the porous material comprises further components, further possible linkages are, for example, urethane groups formed by reaction of isocyanate groups with alcohols or phenols.

The components (a1) and (a2) are preferably linked to an extent of at least 50 mol % by urea groups —NH—CO—NH— and/or via isocyanurate linkages in the porous material. The components (a1) and (a2) are preferably linked to an extent of from 50 to 100 mol % by urea groups and/or via isocyanurate linkages in the porous material, in particular from 60 to 100 mol %, very particularly preferably from 70 to 100 mol %, in particular from 80 to 100 mol %, for example from 90 to 100 mol %.

The balance to 100 mol % are present as further linkages, with such further linkages being known per se to a person skilled in the art from the field of isocyanate polymers. Examples which may be mentioned are ester, urea, biuret, allophanate, carbodiimide, isocyanurate, uretdione and/or urethane groups.

The determination of the mol % of the linkages of the monomer building blocks in the porous material is carried out by means of NMR spectroscopy (nuclear magnetic resonance) in the solid or in the swollen state. Suitable methods of determination are known to those skilled in the art.

The density of the porous material which can be obtained according to the invention is usually from 20 to 600 g/l, preferably from 50 to 500 g/l and particularly preferably from 70 to 200 g/l.

The process of the invention gives a coherent porous material and not only a polymer powder or particles. Here, the three-dimensional shape of the resulting porous material is determined by the shape of the gel which is in turn determined by the shape of the gelling apparatus. Thus, for example, a cylindrical gelling vessel usually gives an approximately cylindrical gel which can then be dried to give a porous material having a cylindrical shape. However, it is in principle also possible to produce a powder, for example by milling the porous material.

The aerogels and xerogels which can be obtained according to the invention have a low thermal conductivity, a high porosity and a low density combined with high mechanical stability. In addition, the porous materials have a small average pore size. The combination of the abovementioned properties allows the materials to be used as insulation material in the field of thermal insulation, in particular for applications in the vacuum sector where a very low thickness of vacuum panels is preferred, for example in refrigeration appliances or in buildings. Thus, the use in vacuum insulation panels, in particular as core material for vacuum insulation panels, is preferred. In addition, use of the aerogels and xerogels of the invention as insulation material is preferred.

Furthermore, uses at pressures of from 1 to 100 mbar and in particular from 10 to 100 mbar are possible because of the low thermal conductivity of the aerogels and xerogels which can be obtained according to the invention. The property profile of the aerogels and xerogels which can be obtained according to the invention opens up, in particular, uses in which a long life of the vacuum panels is desired and in which the thermal conductivity remains low after many years even in the even of a pressure increase of about 2 mbar per year, for example at a pressure of 100 mbar. The aerogels and xerogels which can be obtained according to the invention have advantageous thermal properties and also advantageous materials properties such as simple proccessability and high mechanical stability, for example low brittleness.

EXAMPLES

The density p of the porous gel in the unit g/ml was calculated according to the formula ρ=m/(π*r²)*h, where m is the mass of the porous gel, r is the radius (half diameter) of the porous gel and h is the height of the porous gel.

The porosity in the unit % by volume was calculated according to the formula P=(V_(i)/(V_(i)+V_(s)))*100% by volume, where P is the porosity, V_(i) is the specific volume of the porous gel in ml/g and is calculated according to V_(i)=1/ρ. V_(s) is the specific volume in ml/g of the test specimen. The value 1/V_(s)=1.38 g/ml was used as specific volume. This value can be determined by He pycnometry.

The average pore diameter was determined by means of mercury intrusion in accordance with DIN 66133 and is a volume average.

The lambda value was determined by the dynamic hot wire method. Here, a wire which simultaneously serves as heating element and temperature sensor is embedded in the sample. The wire is heated at a constant electric power. Further details of this measurement method may be found, for example, in H.-P. Ebert et al., High Temp.—High Press., 1993, 25, 391-402.

The following compounds were used:

Component a1:

Oligomeric MDI (Lupranat® M200) having an NCO content of 30.9 g per 100 g in accordance with ASTM D-5155-96 A, a functionality in the region of three and a viscosity of 2100 mPa·s at 25° C. in accordance with DIN 53018 (hereinafter “compound M200”).

Oligomeric MDI (Lupranat® M20) having an NCO content of 31.5 g per 100 g in accordance with ASTM D-5155-96 A, a functionality in the region of three and a viscosity of 210 mPa·s at 25° C. in accordance with DIN 53018 (hereinafter “compound M20”).

Component a2:

3,3′,5,5′-Tetraethyl-4,4′-diaminodiphenylmethane (hereinafter “MDEA”) 3,3′,5,5′-Tetramethyl-4,4′-diaminodiphenylmethane (hereinafter “MDMA”) 4,4′-Diaminodiphenylmethane (hereinafter “MDA”).

Catalysts:

Lupragen® N600, N,N′,N″-tris(dimethylaminopropyl)-s-hexahydrotriazine (hereinafter “compound N600”)

Example 1

1.8 g of the compound M200 were dissolved while stirring at 20° C. in 10.5 g of acetone in a glass beaker. 1.6 g of MDEA (a2-1) and 0.1 g of compound N600 were dissolved in 11 g of acetone in a second glass beaker. The two solutions from step (a) were mixed. This gave a clear, low-viscosity mixture. The mixture was allowed to stand at room temperature for 24 hours to effect curing. The gel was subsequently taken from the glass beaker and the liquid (acetone) was removed by drying at 20° C. for 7 days.

The material obtained had a bimodal pore size distribution having pore diameters in the range from 0.4 μm to 2 μm. The porosity was 83% by volume with a corresponding density of 185 g/l.

Example 2

1.8 g of the compound M200 were dissolved while stirring at 20° C. in 10.5 g of acetone in a glass beaker. 1.6 g of MDEA (a2-1) and 0.2 g of compound N600 were dissolved in 10.8 g of acetone in a second glass beaker. The two solutions from step (a) were mixed. This gave a clear, low-viscosity mixture. The mixture was allowed to stand at room temperature for 24 hours to effect curing. The gel was subsequently taken from the glass beaker and the liquid (acetone) was removed by drying at 20° C. for 7 days.

The material obtained had a bimodal pore size distribution having pore diameters in the range from 0.3 μm to 2 μm. The porosity was 83% by volume with a corresponding density of 190 g/l.

Example C3

1.6 g of the compound M200 were dissolved while stirring at 20° C. in 10.5 g of acetone in a glass beaker. 1.6 g of MDEA were dissolved in 11 g of acetone in a second glass beaker. The two solutions from step (a) were mixed. This gave a clear, low-viscosity mixture. The mixture was allowed to stand at room temperature for 24 hours to effect curing. The gel was subsequently taken from the glass beaker and the liquid (acetone) was removed by drying at 20° C. for 7 days.

The material obtained had an average pore diameter of 4 μm. The porosity was 89% by volume with a corresponding density of 135 g/l.

Example C4

1.9 g of the compound M200 were dissolved while stirring at 20° C. in 10.5 g of acetone in a glass beaker. 1.3 g of MDA were dissolved in 11 g of acetone in a second glass beaker. The two solutions from step (a) were mixed. This gave a clear, low-viscosity mixture. The mixture was allowed to stand at room temperature for 24 hours to effect curing. The gel was subsequently taken from the glass beaker and the liquid (acetone) was removed by drying at 20° C. for 7 days.

The material obtained had an average pore diameter of 2.9 μm. The porosity was 87% by volume with a corresponding density of 170 g/l.

Example 5

1.8 g of the compound M20 were dissolved while stirring at 20° C. in 12 g of ethyl acetate in a glass beaker. 1.6 g of MDMA (a2-2) and 0.1 g of compound N600 were dissolved in 12.5 g of ethyl acetate in a second glass beaker. The two solutions from step (a) were mixed. This gave a clear, low-viscosity mixture. The mixture was allowed to stand at room temperature for 24 hours to effect curing. The gel monolith was subsequently taken from the glass beaker and dried in an autoclave by solvent extraction with supercritical CO₂ (described below).

Solvent extraction with supercritical CO₂: the gel monolith was transferred to an autoclave having a capacity of 250 ml. The autoclave was filled with >99% acetone so that the gel monolith was completely covered by acetone and subsequently closed. The monolith was dried in a stream of CO, for 24 hours. The pressure (in the drying system) was in the range from 115 to 120 bar; the temperature was 40° C. At the end, the pressure in the system was reduced in a controlled manner to atmospheric pressure over a period of about 45 minutes at a temperature of 40° C. The autoclave was opened and the porous material obtained was taken out.

The porous material obtained had a density of 136 g/l. The lambda value was 19.8 mW/m*K (pressure: 1 bar). 

1. A process for producing an aerogel or xerogel, which comprises reacting the following components (a1) and (a2) in the presence of a solvent and of at least one catalyst, with the reaction being carried out in the absence of water: (a1) at least one polyfunctional isocyanate, and (a2) at least one polyfunctional substituted aromatic amine (a2-s) having the general formula I

where R¹ and R² can be identical or different and are each selected independently from among hydrogen and linear or branched alkyl groups having from 1 to 6 carbon atoms and all substituents Q¹ to Q⁵ and Q¹′ to Q⁵′ are identical or different and are each selected independently from among hydrogen, a primary amino group and a linear or branched alkyl group having from 1 to 12 carbon atoms, where the alkyl group can bear further functional groups, with the proviso that the compound having the general formula I comprises at least two primary amino groups, where at least one of Q¹, Q³ and Q⁵ is a primary amino group and at least one of Q¹′, Q³′ and Q⁵′ is a primary amino group, and Q², Q⁴, Q²′ and Q⁴′ are selected so that the compound having the general formula I has at least one linear or branched alkyl group, which can bear further functional groups, having from 1 to 12 carbon atoms in the α position relative to at least one primary amino group bound to the aromatic ring, and optionally at least one further polyfunctional aromatic amine (a2-u) which differs from the amines (a2-s) having the general formula I.
 2. The process according to claim 1, wherein Q², Q⁴, Q²′ and Q⁴′ are selected so that the substituted aromatic amine (a2-s) comprises at least two primary amino groups which each have a linear or branched alkyl group having from 1 to 12 carbon atoms, which can bear further functional groups, in the α position.
 3. The process according to claim 1 or 2, wherein the amine component (a2) comprises at least one compound (a2-s) selected from the group consisting of 3,3′,5,5′-tetraalkyl-4,4′-diaminodiphenylmethane, 3,3′,5,5′-tetraalkyl-2,2′-diaminodiphenylmethane and 3,3′,5,5′-tetraalkyl-2,4′-diaminodiphenylmethane, where the alkyl groups in the 3,3′,5 and 5′ positions can be identical or different and are selected independently from among linear or branched alkyl groups which have from 1 to 12 carbon atoms and can bear further functional groups.
 4. The process according to one or more of claims 1 to 3, wherein the alkyl groups of the polyfunctional aromatic amines (a2-s) having the general formula I are selected from among methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl and tert-butyl.
 5. The process according to one or more of claims 1 to 4, wherein the polyfunctional aromatic amines (a2-s) having the general formula I are 3,3′,5,5′-tetraalkyl-4,4′-diaminodiphenylmethanes, preferably 3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane and/or 3,3′,5,5′-tetramethyl-4,4′-diaminodiphenylmethane.
 6. The process according to one or more of claims 1 to 5, wherein the amount of component (a1) used is from 20 to 80% by weight and that of component (a2) is from 20 to 80% by weight, in each case based on the total weight of the components (a1) and (a2), which is 100% by weight.
 7. The process according to one or more of claims 1 to 6, wherein the amount of component (a1) used is from 35 to 68% by weight and that of the component (a2) is from 32 to 65% by weight, in each case based on the total weight of the components (a1) and (a2), which is 100% by weight.
 8. The process according to one or more of claims 1 to 7, wherein component (a2) consists exclusively of compounds of the type (a2-s) having the general formula I.
 9. The process according to one or more of claims 1 to 8, wherein the reaction is carried out in the presence of at least one tertiary amine as catalyst.
 10. The process according to one or more of claims 1 to 9, which comprises: a) provision of the components (a1) and (a2), the catalyst and the solvent defined in claims 1 to 8, b) reaction of the components (a1) and (a2) in the presence of the solvent and the catalyst to form a gel and c) drying of the gel obtained in the preceding step.
 11. The process according to claim 10, wherein the components (a1) and (a2) are provided separately, in each case in a partial amount of the solvent.
 12. The process according to claim 10 or 11, wherein the drying of the gel obtained is carried out by converting the liquid comprised in the gel into the gaseous state at a temperature and a pressure below the critical temperature and the critical pressure of the liquid comprised in the gel.
 13. The process according to claim 10 or 11, wherein the drying of the gel obtained is carried out under supercritical conditions.
 14. A porous material which can be obtained by the process according to one or more of claims 1 to
 13. 15. The use of porous materials according to claim 14 as insulation material or for vacuum insulation panels. 